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Um, the Sentinel-1 sensor is C-band SAR (radar) data, not spectral. Given that NDVI is a spectral index of photosynthetically active radiation using the ratio between Red and NIR spectra, what makes you think that you can get NDVI from SAR polarisation? There is absolutely no way to derive NDVI from SAR data!

Please, go research some basics on active and passive remote sensing as well as spectral indices. In your case, even a Google search will provide a starting point.